The present invention is in the field of photochromic glasses and particularly relates to photochromic glasses exhibiting surface coloration produced by reducing atmosphere heat treatments, and methods for modifying the surface coloration of such glasses.
The development of surface coloration in silver halide-containing photochromic glasses by reducing atmosphere heat treatments, i.e., by heating the glasses under reducing conditions, is disclosed in U.S. Pat. Nos. 3,892,582 and 3,920,463. Such development is thought to involve the partial reduction of Ag.sup.+ in the glass to metallic silver, with the metallic silver acting to provide color centers in the glass which absorb light in the 430-460 nm range, thereby imparting a yellow-to-brown color to the glass.
Glasses which exhibit selective tinting or coloration, i.e., coloration only at specific locations or colored regions forming a design, are of interest for a number of applications. Even in the opthalmic field, opthalmic lenses referred to as gradient lenses which are tinted at the top of the lens and clear at the bottom, are finding commercial acceptance.
If the field of photochromic glasses, efforts have been directed toward the development of lenses which photochromically darken in the top portion only, thus providing a gradient darkening behavior. Examples of methods for producing such lenses are found in U.S. Pat. Nos. 4,036,624, 4,062,490 and 4,160,655.
A particularly desirable product would be a photochromic ophthalmic lens exhibiting a fixed tint gradient in combination with uniform photochromic darkening behavior. Such a lens would darken uniformly in bulk upon exposure to actinic radiation and fade in the absence thereof, and would additionally have an invariant fixed tint gradient or other selectively applied color pattern superimposed on the lens which would not change in response to changes in lighting conditions. In this way a lens exhibiting a photochromic darkening in all lens portions and also a gradient which would always be apparent to the observer, could be provided. Important requirements of such a lens would be relatively uniform photochromic darkening capability, a fixed-tint gradient or other selective coloring pattern, and freedom from refractive power aberrations caused by the lens fabrication procedure. Of course, processes risking the introduction of variations in surface smoothness or curvature which could affect the optical properties of the lens would be unacceptable for both sunglass and prescription ophthalmic lens products.